HSL: click a colour chip to open its panel, the way PICK does

The colour band chips only aimed the mixer before; now they open the same
panel the eyedropper opens, seeded with the colour the chip is named after
(hexToRgb turns that readout back into the RGB bytes the picker stores).
A pick already on the photo keeps its spot — only a mixer with nowhere to
hang takes the middle of the frame.

The HSL column itself is untouched: 8 chips, the rule, IMAGE,
HUE/SAT/LUM, the readout and RESET all stay where they were.

docker/frontend/src/ui/ToolRail.tsx

@@ -72,7 +72,10 @@ export function ToolRail({
           }}
         >
           <span className="glyph" aria-hidden="true">{entry.glyph}</span>
-          {t(entry.label)}
+          {/* A box, not a bare text node: on a phone the glyph is display:none
+              and the name would be the button's only child — an anonymous flex
+              item, which is the one thing iOS stops painting until a touch. */}
+          <span className="rail-label">{t(entry.label)}</span>
         </button>
       ))}
       {/* The build this page came off, at the foot of the rail: an image on the